RF communication terminals normally employ an RF transmitter to generate RF signals, a receiver to receive RF signals and an antenna to send radiated signals produced by the transmitter over-the-air to another terminal and to receive signals sent over-the-air from another terminal for delivery to the receiver. The transmitter normally includes an RF power amplifier, ‘RFPA’, to amplify the RF signals before they are delivered to the antenna for transmission.
It is desirable for the RF transmitter to be linear, i.e. for the RFPA to produce a power amplification which is a linear function of the power of the input signal provided to it, in order to prevent distortion of the input signal and to minimize adjacent channel interference. Many RF transmitters include at least one control loop such as a Cartesian loop to provide linearization of the RFPA of the transmitter.
The control loop may be operated in a training mode to set a suitable strength level (of the baseband signal delivered along the forward path) which in operation does not cause compression of the RFPA.
The control loop may include a loop filter which may include one or more filter stages. One purpose of the loop filter is to constrain the bandwidth of the loop to ensure stability of the loop. In filter analysis, it well known to define such a filter in terms of the transfer function of the filter, especially parameters known as the poles and zeros which are obtained from the transfer function of the filter. For example, the transfer function H(s) of an LTI filter used as a loop filter may be defined as H(s)=Y(s)/X(s) where the terms Y(s) and X(s) are polynomial expressions which can be factorised; the multiplying factors of the factorised expressions can be written in the form, s-qi where i is an integer, 1, 2, 3 . . . . The (possibly complex) numbers qi are the roots of the polynomial. When s is set to the value of any of these roots of the numerator polynomial term Y(s) which results in the transfer function evaluating to zero, the root in question is denoted as a ‘zero’. When s is set to the value of any of the roots of the denominator polynomial term X(s) which results in the transfer function approaching infinity, the root in question is denoted as a ‘pole’. The concept of zeros and poles will of course be familiar to those skilled in the art of designing filters.
The loop filter used in a control loop such as a Cartesian loop in a linear RF transmitter is normally designed to have a first pole at a low frequency and a second pole and a zero at higher frequencies. The precise positions in frequency of the first pole, the second pole and the zero are selected according to the properties of the RF signal that has to be transmitted by the transmitter. In some transmitters, the selection of these positions can lead to a serious problem during level training described above. The loop filter may no longer be a perfect integrator and this can cause difficulty in identifying during level training the appropriate strength level needed to avoid compression of the RFPA.
Thus, there exists a need for a linear RF transmitter, for use in mobile communications, which addresses at least some of the shortcomings of past and present techniques and/or procedures employed for level training in such transmitters.
Skilled artisans will appreciate that items shown in the drawings are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the items may be exaggerated relative to other items to assist understanding of various embodiments. In addition, the description and drawings do not necessarily require the order illustrated. Apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the various embodiments so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Thus, it will be appreciated that for simplicity and clarity of illustration, common and well-understood items that are useful or necessary in a commercially feasible embodiment may not be depicted in order to facilitate a less obstructed view of these various embodiments.